Method for controlling an internal combustion engine

ABSTRACT

A method and a device for controlling an internal combustion engine, including a sensor for detecting a pressure variable characterizing the pressure of the air supplied to the internal combustion engine. The performance reliability of the sensor is monitored and a substitute signal is used in case of a fault. To determine the substitute signal, a static substitute value is defined on the basis of variables that characterize the operating state of the internal combustion engine. The static substitute value is filtered by a filter having a delay component, in order to generate the substitute signal.

FIELD OF THE INVENTION

The present invention relates to a method and a device for controllingan internal combustion engine.

BACKGROUND INFORMATION

A method and a device for controlling an internal combustion engine arediscussed in the German Published Patent Application No. 40 32 451. Asensor may be used for detecting a pressure variable that maycharacterize the pressure of the air supplied to the internal combustionengine. The performance reliability of the sensor may be monitored and asubstitute signal may be used in case of a fault. If a fault is present,the output signal of a second sensor may be used as a substitute value.This method may require an additional sensor.

SUMMARY OF THE INVENTION

By using a static substitute value based on quantities characterizingthe operating state of the internal combustion engine in determining thesubstitute signal, a substitute value may be provided in a simple andcost-effective manner. The static substitute value, defined forgenerating the substitute signal, may be filtered by a filter having adelay component. As a result of such filtering, dynamic effects may betaken into account. For instance, the charge-air pressure may have adelayed reaction in response to a change in the fuel quantity and/orrotational speed. Therefore, a precise simulation may only be providedif the output variable of the simulation has a delayed change inresponse to a change in the input variables.

A further improvement in the simulation may result if the responsecharacteristic of the filter is specifiable as a function of operatingcharacteristics.

The rotational speed of the internal combustion engine and/or the timederivative of the pressure variable may be suitable in this context. Atdifferent speeds, different time constants may be selected for thefilter. Correspondingly, different time constants may be selected atrising and falling speeds. In this manner, the simulation may be moreprecisely adapted to the actual behavior of the signal.

A fault of the sensor may be recognized when a change in a variablecharacterizing the fuel quantity to be injected does not result in achange in the signal. In this manner, a reliable and simple faultdetection may be made provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an examplary embodiment of a system fordetecting the charge-air pressure.

FIG. 2 shows a detailed representation of a monitoring of the charge-airpressure.

FIG. 3 shows a block diagram representing a formation of a substitutevalue for the charge-air pressure.

DETAILED DESCRIPTION

In the following, an exemplary method of the present invention isdescribed using a charge-air pressure sensor. However, the presentinvention may not be limited to this application. The exemplary methodaccording to the present invention may be used with sensors in which achange in an operating characteristic results in a corresponding changein the sensor's output signal. In particular, the exemplary method inaccordance with the present invention may also be used in a sensor fordetecting the air mass or a quantity correlated to the charge-airpressure or a quantity characterizing the charge-air pressure. Inparticular, the exemplary method may be used with a sensor for detectingair mass.

In FIG. 1, a sensor for detecting charge-air pressure and the associatedanalog/digital converter are denoted by the reference numeral 100. Thesensor supplies a signal UP, which corresponds to the charge-airpressure, to a characteristic curve 110. There, this quantity isconverted into a signal PR, which in turn is transmitted to a filter120. Via a first switching arrangement 130, output signal P of filter120 arrives at control 140, which then further processes this signal inorder to appropriately control the internal combustion engine oractuators mounted on the internal combustion engine.

At the second input of first switching arrangement 130, an output signalPS of a simulation 135 is available. This simulation 135 calculates asimulated charge-air pressure PS on the basis of different variables.

Switching arrangement 130 is controllable by a first monitor 150. Thismeans that, in case of a recognized fault, the first monitor switchesfirst switching arrangement 130 into such a position that output signalPS of simulation 135 arrives at control 140. First monitor 150 evaluatessignals from various sensors 160, which characterize, for example, fuelquantity QK to be injected and/or rotational speed N of the internalcombustion engine. Furthermore, output signal PR of characteristics map110 may be evaluated for the purpose of fault monitoring. Alternativelyor in addition, output signal P of filter 120 and output signal UP ofthe A/D converter of sensor 100, respectively, may also be directlyprocessed.

A further exemplary embodiment is depicted by a dotted line. In thiscase, a second switching arrangement 170 is arranged between firstswitching arrangement 130 and control 140, second switching arrangement170 being controlled by a second monitor 180. If a fault is recognized,second monitor 180 controls switching arrangement 170 in such a mannerthat output signal PA of a delay 175 arrives at control 140. This hasthe effect that, if a fault has been recognized, the value lastrecognized as faultless will continue to be used.

The sensor's output signal, which is provided by an A/D converter, isconverted by characteristics map 110 into a quantity PR corresponding tothe pressure. After the first monitor and/or the second monitor has/haveevaluated the different signals, various faults are recognized.

By appropriately controlling first switching arrangement 130 and/orsecond switching arrangement 170, a substitute value PS or an earlierstored value PA may be used as a substitute value by control 140 in thecontrol of the internal combustion engine in case of a fault. For thispurpose, delay 175 stores the value last recognized as faultless. Thisold value PA stored in delay 175 is then used in the control of theinternal combustion engine.

Different faults may be recognized by the first monitor and/or thesecond monitor. For instance, a signal-range check may be provided at aminimum and/or a maximum value for signal UP and signal PR,respectively. Furthermore, a plausibility check by a further sensor,such as an atmospheric air-pressure sensor, may be implemented givenparticular operating conditions.

In addition, in accordance with the present invention, a plausibilitytest may be implemented using the injection quantity and/or some otheroperating characteristic having an important influence on charge-airpressure. This plausibility test may be performed in such a manner thata fault is recognized when a change in the operating characteristic doesnot result in a corresponding change in the output quantity of thesensor.

A quantity characterizing the injected fuel quantity may be used as anoperating characteristic. For this purpose, a setpoint value for thefuel quantity to be injected and/or a controlled variable used in thecontrol of a fuel-quantity determining actuator may be used. Forinstance, the control duration of an electromagnetic valve or apiezomagnetic valve may be suitable. This monitoring is depicted in moredetail in FIG. 2.

If a corresponding fault is recognized, first switching arrangement 130switches to simulated substitute signal PS. This means that thefunctioning of the sensor is monitored and substitute signal PS is usedin case of a fault. The substitute signal is detected using quantitiescharacterizing the operating state of the internal combustion engine.The value formed in such a manner is additionally filtered by a filterhaving a delaying component. FIG. 3 shows in detail how the substitutevalue is formed.

First monitor 150 is shown in more detail in FIG. 2 via an example. Inparticular operating states, it may occur that charge-air pressure UPremains constant, although the actual air-charge pressure is changing.Such a fault may also be referred to as freezing of the sensor. Todetect this fault, the fault monitoring shown in FIG. 2 is implemented.

According to the present invention, the monitoring is only performed incertain operating states. Given such an operating state, in which thecharge-air temperature is below a threshold value TLS, and therotational speed and the fuel quantity to be injected are within certainvalue ranges, the current quantity and the currently existing charge-airpressure are stored as old values QKA and PA, respectively, afterchanging the preceding sign when changing the fuel quantity to beinjected. A time meter is activated simultaneously. After a delay haselapsed, difference QKD between the old stored value QKA and the nowcurrent value QK of the injection quantity is formed. In a correspondingmanner, change PD of the pressure is also determined during this delay.

If the amount of the difference between the fuel quantity values isgreater than a threshold value QKDS, the amount of the change inair-charge pressure may also need to be greater than a threshold valuePDS. If this is not the case, a fault is detected.

In FIG. 2, an exemplary embodiment of such a monitoring device isrepresented via example. Output signal TL of a temperature sensor 160 c,which provides a signal corresponding to the charge-air temperature, issupplied to a first comparator 200. In addition, a threshold value TLSis conveyed to comparator 200 by a threshold input 205. Comparator 200applies a corresponding signal to an AND gate 210. The output signal ofa characteristics map 220 is transmitted to a second comparator 230, atwhose input rotational speed signal N of a speed sensor 160 a isavailable. In addition, characteristics map 220 processes a quantity QK,characterizing the fuel quantity to be injected and which may beprovided by a device for controlling injected fuel-quantity 160 b.Furthermore, a threshold value BPS is transmitted to comparator 230 by athreshold input 235. Comparator 230 also applies a corresponding signalto AND gate 210.

Quantity QK also arrives at a preceding-sign detector 250 and a filter260. The output signal of preceding-sign detector 250 is applied to atime counter 270 as well as a first memory 262 and a second memory 265.The output signal of filter 260, having a positive preceding sign, firstof all arrives directly at a node 285 and then, via first memory 262,with a negative preceding sign, at a second input of node 285. Node 285applies a quantity QKD to a switching arrangement 275. The output signalQKD of switching arrangement 275 arrives at a third comparator 280, atwhose second input, output signal QKDS of a threshold input 285 isavailable. Evaluator 240 also receives the output signal of comparator280.

Output signal P of filter 120, with a positive preceding sign, firstarrives directly at a node 287 and then, via second memory 265, with anegative preceding sign, at a second input of node 287. Node 287 appliesa quantity PD to switching arrangement 276. The output signal PD ofswitching arrangement 276 arrives at a fourth comparator 290, at whosesecond input output signal PDS of a threshold input 295 is available.The output signal of comparator 290 is also applied to evaluator 240.

First comparator 200 compares measured charge-air temperature TL withthreshold value TLS. If measured charge-air temperature TL is lower thanthreshold value TLS, a corresponding signal arrives at AND gate 210.Based on at least the rotational speed and/or the fuel quantity to beinjected, characteristics map 220 forms a characteristic value, whichcharacterizes the operating state of the internal combustion engine.This characteristic value is compared in comparator 230 with thresholdvalue BTS. If the characteristic value for the operating state isgreater than threshold value BPS, a corresponding signal is transmittedto AND gate 210. If both conditions are satisfied, i.e. if the airtemperature is lower than threshold value TLS, and given particularoperating states, monitoring may be provided.

This logic unit, having comparators 200 and 230, threshold inputs 205and 235, characteristics map 220 and the AND gate, has the effect ofimplementing the monitoring of the sensor signal as a function of thepresence of certain operating states. Monitoring is only implemented ifthe air temperature is lower than a threshold value and if certainvalues for the rotational speed and/or the fuel quantity to be injectedare given.

Preceding-sign detector 250 examines whether a preceding-sign change ispresent in the change of the fuel quantity, i.e. it is examined whetherthe time derivative of the fuel quantity to be injected includes a zerocrossing. If this is the case, the current values of the fuel quantityto be injected are stored as old value QKA in memory 262.Correspondingly, the current value of the pressure is stored as oldvalue PA in second memory 265. The fuel quantity to be injected may befiltered by filter 260 prior to storing.

Time counter 270 is activated simultaneously with the detected change inthe preceding sign. Based on current value QK and old value QKA of thefuel quantity to be injected, a differential value QKD is formed in node285, which indicates the change in the fuel quantity since the lastpreceding-sign change. Similarly, a corresponding differential value PDis formed in node 287 for the pressure, which characterizes the changein charge-air since the last change in the preceding sign. If the timecounter has elapsed, i.e., if a certain delay since the last change inpreceding signs has been satisfied, differential signal QKD is comparedby comparator 280 to a threshold value QKDS. Differential pressure PD iscompared in a similar manner in node 290 to a corresponding thresholdvalue PDS. If the two values for the difference in fuel quantity QKD anddifferential pressure PD are in each case greater than the thresholdvalue, the device does not identify a fault. If only the difference offuel quantity QKD is greater than the threshold value, and value PD forthe pressure is lower than threshold value PDS, the device identifies afault. In this case, monitor 150, i.e. evaluator 240, inputs anappropriate signal for controlling switchover 130.

The method described herein is an exemplary embodiment. Other exemplaryembodiments may also be provided. For example, other program steps mayperform the check test. A fault may be identified when a change in anoperating characteristic occurs, such as the fuel quantity to beinjected, does not result in a corresponding change in charge-airpressure. If, after a preceding-sign change in the change of the fuelquantity, a change in the fuel quantity correlates with a change in thepressure variable, no fault is present.

Other variables characterizing the fuel quantity to be injected may beused instead of the fuel quantity, that is, those variables that are afunction of the fuel quantity or are determined as a function of thefuel quantity. For instance, a load variable may be used, such as, forexample, a torque variable and/or a control variable of a fuel-quantitycontrol element.

FIG. 3 shows simulation 135 in more detail. Components already describedin FIG. 1 are identified by corresponding reference numerals. Signal Nof rotational speed sensor 160 a and signal QK regarding the injectedfuel quantity arrive at a characteristics map 300, whose output variablearrives at switching arrangement 130 via a filter 310. Rotational speedN also arrives at filter 310, via a characteristic curve 320 and a node330. At the second input of node 330, the output signal of apreceding-sign detector 340 is available.

A value for charge-air pressure P is stored in characteristics map 300as a function of the operating state of the internal combustion engine.This stored value corresponds to charge-air pressure in the staticstate. In order to be able to take dynamic states into account, filterarrangement 310 is provided. This filter arrangement 310 may beconfigured as PT1-filter and may simulate the time characteristic of thepressure when a change occurs in the operating state. The responsecharacteristic of this filter arrangement 310 may be varied as afunction of the operating state of the internal combustion engine.Characteristic curve 320 is provided particularly for this purpose, inwhich, as a function of rotational speed N, a variable is stored thatdetermines the response characteristic of filter arrangement 310.

The time constant for the filter may be smaller at large rotationalspeeds, and larger in case of low rotational speed. The responsecharacteristic is determined by preceding-sign detector 340, whichinputs a correction value for correcting the output signal ofcharacteristic curve 320 as a function of the preceding sign of thepressure change. Preceding-sign detector 340 determines whether thepressure is rising or falling.

A larger time constant may be chosen for the filter when the pressure isrising than when the pressure is falling.

The output signal of characteristics map 300 and also the output signalof filter arrangement 310 may be used as input variables in detectingthe preceding sign. Using a specifiable value, an additive and/or amultiplicative correction of the output signal, which is a function ofspeed of characteristics map 320, is implemented.

In accordance with the present invention, the response characteristic offilter 310 is specified as a function of the speed of the internalcombustion engine and the change direction of the pressure.

What is claimed is:
 1. A method for controlling an internal combustionengine including a sensor, the method comprising: detecting with thesensor a pressure variable characterizing a pressure of air supplied tothe internal combustion engine; monitoring a performance reliability ofthe sensor for a fault; defining a static substitute value based on atleast one variable characterizing an operating state of the internalcombustion engine; filtering the static substitute value with a filterhaving a delay component; determining a substitute signal using thefiltered static substitute value; using the substitute signal if thefault occurs; and specifying a response characteristic of the filter asa function of an operating parameter.
 2. The method of claim 1, whereinthe response characteristic is specifiable as a function of a speed. 3.The method of claim 1, wherein the response characteristic isspecifiable based on a time derivative of the pressure variable.
 4. Themethod of claim 1, wherein the at least one variable characterizing theoperating state of the internal combustion engine include at least oneof a rotational speed and a fuel quantity to be injected.
 5. The methodof claim 1, wherein the substitute signal is used if an output signal ofthe sensor is recognized as being the fault.
 6. A method for controllingan internal combustion engine including a sensor, the method comprising:detecting with the sensor a pressure variable characterizing a pressureof air supplied to the internal combustion engine; monitoring aperformance reliability of the sensor for a fault; defining a staticsubstitute value based on at least one variable characterizing anoperating state of the internal combustion engine; filtering the staticsubstitute value with a filter having a delay component; determining asubstitute signal using the filtered static substitute value; using thesubstitute signal if the fault occurs; and recognizing a signal as beingthe fault if a change in a variable characterizing a fuel quantity to beinjected does not result in a change in the signal.
 7. A device forcontrolling an internal combustion engine, the device comprising: asensor to detect a pressure variable characterizing a pressure of airsupplied to the internal combustion engine; an monitoring arrangement tomonitor a performance reliability of the sensor, and being operable touse a substitute signal if a fault occurs; a determining arrangement todetermine a substitute signal, the determining arrangement including afilter having a delay component to generate the substitute signal byfiltering a static substitute value defined on a basis of at least onevariable characterizing an operating state of the internal combustionengine; and an arrangement for specifying a response characteristic ofthe filter as a function of an operating parameter.